Handler handler =new Handler(){
  @Override
  public void handleMessage(Message msg) {
   super.handleMessage(msg);
   //這里進行一些UI操作等處理
  }

   new Thread(new Runnable() {
   @Override
   public void run() {
    Message message = Message.obtain();
    ........
    handler.sendMessage(message);
   }
  });
 };

public Handler() {
 this(null, false);
}
//他會調(diào)用本類中的如下構(gòu)造函數(shù)
public Handler(Callback callback, boolean async) {
  if (FIND_POTENTIAL_LEAKS) {
   final Class<? extends Handler> klass = getClass();
   if ((klass.isAnonymousClass() || klass.isMemberClass() || klass.isLocalClass()) &&
     (klass.getModifiers() & Modifier.STATIC) == 0) {
    Log.w(TAG, "The following Handler class should be static or leaks might occur: " +
     klass.getCanonicalName());
   }
  }

  mLooper = Looper.myLooper();
  if (mLooper == null) {
   throw new RuntimeException(
    "Can't create handler inside thread that has not called Looper.prepare()");
  }
  mQueue = mLooper.mQueue;
  mCallback = callback;
  mAsynchronous = async;
 }

public static void prepare() {
  prepare(true);
}
//將looper保存到ThreadLocal中，這里可以把ThreadLocal理解為一個以當前線程為鍵的Map，所以一個線程中只會有一個looper
private static void prepare(boolean quitAllowed) {
 if (sThreadLocal.get() != null) {
  throw new RuntimeException("Only one Looper may  be created per thread");
  }
 sThreadLocal.set(new Looper(quitAllowed));
}
//我們看到在new Looper(quitAllowed)中，創(chuàng)建了一個消息隊列MessageQueen
private Looper(boolean quitAllowed) {
 mQueue = new MessageQueue(quitAllowed);
 mThread = Thread.currentThread();
}

public boolean sendMessageAtTime(Message msg, long uptimeMillis) {
  MessageQueue queue = mQueue;
  if (queue == null) {
   RuntimeException e = new RuntimeException(
     this + " sendMessageAtTime() called with no mQueue");
   Log.w("Looper", e.getMessage(), e);
   return false;
  }
  return enqueueMessage(queue, msg, uptimeMillis);
 }

private boolean enqueueMessage(MessageQueue queue, Message msg, long uptimeMillis) {
  msg.target = this;
  if (mAsynchronous) {
   msg.setAsynchronous(true);
  }
  return queue.enqueueMessage(msg, uptimeMillis);
 }

//MessageQueen的enqueueMessage方法
 boolean enqueueMessage(Message msg, long when) {
  if (msg.target == null) {
   throw new IllegalArgumentException("Message must have a target.");
  }
  if (msg.isInUse()) {
   throw new IllegalStateException(msg + " This message is already in use.");
  }

  synchronized (this) {
   if (mQuitting) {
    IllegalStateException e = new IllegalStateException(
      msg.target + " sending message to a Handler on a dead thread");
    Log.w(TAG, e.getMessage(), e);
    msg.recycle();
    return false;
   }

   msg.markInUse();
   msg.when = when;
   Message p = mMessages;
   boolean needWake;
   if (p == null || when == 0 || when < p.when) {
    // New head, wake up the event queue if blocked.
    msg.next = p;
    mMessages = msg;
    needWake = mBlocked;
   } else {
    // Inserted within the middle of the queue. Usually we don't have to wake
    // up the event queue unless there is a barrier at the head of the queue
    // and the message is the earliest asynchronous message in the queue.
    needWake = mBlocked && p.target == null && msg.isAsynchronous();
    Message prev;
    for (;;) {
     prev = p;
     p = p.next;
     if (p == null || when < p.when) {
      break;
     }
     if (needWake && p.isAsynchronous()) {
      needWake = false;
     }
    }
    msg.next = p; // invariant: p == prev.next
    prev.next = msg;
   }

   // We can assume mPtr != 0 because mQuitting is false.
   if (needWake) {
    nativeWake(mPtr);
   }
  }
  return true;
 }

public static void loop() {
  final Looper me = myLooper();
  if (me == null) {
   throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");
  }
  final MessageQueue queue = me.mQueue;

  // Make sure the identity of this thread is that of the local process,
  // and keep track of what that identity token actually is.
  Binder.clearCallingIdentity();
  final long ident = Binder.clearCallingIdentity();

  for (;;) {
   //不斷從消息隊列中取出msg
   Message msg = queue.next(); // might block
   if (msg == null) {
    // No message indicates that the message queue is quitting.
    return;
   }

   // This must be in a local variable, in case a UI event sets the logger
   Printer logging = me.mLogging;
   if (logging != null) {
    logging.println(">>>>> Dispatching to " + msg.target + " " +
      msg.callback + ": " + msg.what);
   }
   //將msg交由handler處理
   msg.target.dispatchMessage(msg);

   if (logging != null) {
    logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);
   }

   // Make sure that during the course of dispatching the
   // identity of the thread wasn't corrupted.
   final long newIdent = Binder.clearCallingIdentity();
   if (ident != newIdent) {
    Log.wtf(TAG, "Thread identity changed from 0x"
      + Long.toHexString(ident) + " to 0x"
      + Long.toHexString(newIdent) + " while dispatching to "
      + msg.target.getClass().getName() + " "
      + msg.callback + " what=" + msg.what);
   }

   msg.recycleUnchecked();
  }
 }

public void dispatchMessage(Message msg) {
  if (msg.callback != null) {
   handleCallback(msg);
  } else {
   if (mCallback != null) {
    if (mCallback.handleMessage(msg)) {
     return;
    }
   }
   handleMessage(msg);
  }
 }

//當我們通過這種方式創(chuàng)建handler時，dispatchMessage中的mCallback就不為null
 public Handler(Callback callback) {
  this(callback, false);
 }
//Callback是一個接口，里面正好也有我們需要的handleMessage(Message msg)，dispatchMessage中的 if (mCallback != null) 語句內(nèi)的內(nèi)容，就是我們需要重寫的handleMessage(Message msg)方法
 public interface Callback {
  public boolean handleMessage(Message msg);
 }

//當我們調(diào)用handler.post()方法執(zhí)行異步任務(wù)時
 public final boolean post(Runnable r)
 {
  return sendMessageDelayed(getPostMessage(r), 0);
 }
//getPostMessage(r)這個方法中我們看到給m.callback賦值了，就是我們傳入的runnable接口
 private static Message getPostMessage(Runnable r) {
  Message m = Message.obtain();
  m.callback = r;
  return m;
 }
//最后在handleCallback方法中我們執(zhí)行了它的run方法,這也就解釋了為什么在子線程中可以用handler.post(Runnable r)更新UI
 private static void handleCallback(Message message) {
  message.callback.run();
 }